This invention relates to a color display system utilizing an astigmatic deflection field for in-line beam convergence and predistortion of each beam shape for compensation for deflection-induced beam distortion.
In the past convergence of the beams of color picture tubes, such as used in color television receivers, generally has been accomplished by the use of magnetic pole pieces disposed within the neck of a picture tube and energized by external electromagnets driven by convergence correction waveforms at both the line and field scanning rates to effect convergence of the beams as seen on the viewing screen of the picture tube. This is commonly referred to as on-axis dynamic convergence correction. Additionally, sometimes to correct corner misconvergence it has been necessary to utilize further correction waveforms derived by combining the line and field rate waveforms. Obviously, structure of this type is expensive and usually requires the adjustment of many controls to properly converge the beams.
Color picture tubes utilizing coplanar beams horizontally disposed, and particularly in conjunction with vertically disposed phosphor stripes on the viewing screen, have enabled the use of dynamic convergence arrangements which are simpler than the above-described arrangements utilized with picture tubes having delta electron beam configurations and dot phospher elements arranged in triad groupings. It is known that quadrupole magnetic field-producing windings may be utilized in conjunction with the deflection yoke to achieve convergence of the coplanar beams. In general the quadrupole windings must be energized by both line and field rate waveforms and a number of adjustable control elements utilized to achieve the desired convergence of the beams. Alternatively, in addition to a quadrupole winding, the scanning current through the actual deflection windings may be proportioned to achieve convergence, but this approach also requires a number of adjustable controls, all of which add to the cost and complexity of manufacturing and servicing the television receiver.
In U.S. Pat. No. 3,800,176 entitled, "SELF-CONVERGING COLOR IMAGE DISPLAY" and issued Mar. 26, 1974, to the present inventors, a system is described which provides self-convergence of three coplanar beams of a color picture tube without the use of any dynamic convergence apparatus. That patent also discloses that in relatively large viewing screen size picture tubes, such as tubes having a viewing screen diagonal dimension of about 25 inches, it may be desirable to utilize some form of simplified dynamic convergence arrangement to achieve substantial convergence of the beams at all points on the viewing screen.
The self-converging feature of the three in-line beams is provided by producing astigmatic deflection fields, the field generally being a pincushion shaped field produced by the horizontal deflecting components and a barrel shaped field produced by the vertical deflecting components. This astigmatism is produced and controlled by the conductor distribution of the wound components placed about the neck region of the picture tube. This self-converging feature is extremely desirable in that it eliminates or, in the case of simplified dynamic convergence systems, greatly reduces the amount of dynamic convergence circuitry and the attendant set-up and servicing time. However, with picture tubes utilizing wider deflection angles such a 110.degree. and larger viewing screens such as screens having a diagonal measurement of about 63 cm, and whether or not some form of simplified dynamic convergence is utilized in conjunction with the self-converging characteristics of the wound components, the electron beam shape may undesirably distort from a substantially circular spot at the center region of the viewing screen to a horizontal ellipse as a function of distance along the horizontal deflection axis. Under these conditions horizontal resolution of the display system may be impaired to the extent that the reproduced picture is commercially unacceptable.